Anthropogenic climate change is causing rising global temperatures, changes in patterns of precipitation, and, according to many, an increase in the extent and impact of vector-borne infectious disease on human health. This last assertion, however, rests heavily on the assumption that a vector's current distribution is restricted by climatic conditions-as these conditions change, so will the distribution. Consider the blacklegged tick (Ixodes scapularis), which vectors the agents of three emerging infectious diseases in North America: Lyme disease, human granulocytic anaplasmosis, and babesiosis. It has long been hypothesized that the cold, dry winter conditions of northern and interior environments kill overwintering ticks and thus limit the distribution of these vectors. As winters in the Northeast become warmer and wetter, the range of I. scapularis and the diseases it brings with it are expected to increase. But again, this prediction is based on broad-scale correlations between tick abundance and/or mortality and prevailing climatic conditions. It has never been demonstrated that climate actually limits tick survival in nature. We propose to experimentally test whether and how increasing temperature and changing precipitation regimes will influence overwintering mortality of nymphal blacklegged ticks. In order to provide the most accurate, realistic measurements of tick survival, we have devised semi-natural enclosures consisting of soil cores enclosed in nylon mesh inserted into the forest floor. We will use these to study overwintering mortality in the forests of the Institute of Ecosystem Studies, where ticks and tick-borne disease are common, and SUNY College of Environmental Science and Forestry, where ticks are much more rare. In the first year we will track mortality at two-week intervals to determine when mortality rates peak and whether this is associated with extreme weather events. In the second year we will elevate the soil temperature 50C above ambient and prevent new snowfall accumulation in a factorial design to examine how projected climate conditions influence tick survival. This will be the first experimental test of whether and how a changing climate may influence this important vector of zoonotic diseases. PUBLIC HEALTH RELEVANCE Climate change is expected to increase the range and impacts of vector-borne disease on human health, but the mechanism leading to these changes are unclear. This research will provide a definitive, experimental test of how changing climatic conditions might influence the distribution of blacklegged ticks (Ixodes scapularis), the vector of the agents of Lyme disease, human granulocytic anaplasmosis, and babesiosis in North America.